1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine, such as an automotive internal combustion engine, for adjusting a mixture ratio of an air/fuel mixture to be introduced into an engine combustion chamber for maintaining the air/fuel ratio as close as possible to a stoichiometric value. More specifically, the invention relates to an air/fuel ratio control system which can perform a learning process for continuously and cyclically updating the correction coefficient for a basic fuel delivery amount.
2. Description of Background Art
The Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 60-145443 discloses an air/fuel ratio control system with a correction coefficient learning feature. The disclosed system is particularly directed for a so-called L-jectronics type fuel injection internal combustion engine and designed for performing air/fuel ratio control for the engine of this type. As is well known, air/fuel ratio control is performed by adjusting fuel delivery or fuel injection amount so that the air/fuel ratio can be held as close as possible to a stoichiometric value. It is also well known that a basic fuel injection amount Tp is determined generally on the basis of an engine speed indicative parameter and an engine load indicative parameter and is corrected by a plurality of correction factors, such as a cold engine enrichment factor, an acceleration enrichment factor and so forth. The air/fuel ratio is regarded as one of principle correction factor for correcting the basic fuel injection amount based thereon.
As is well known, the air/fuel ratio dependent correction factor is derived on the basis of an oxygen concentration in an exhaust gas. In the air/fuel ratio control system disclosed in the aforementioned Tokkai Showa No. 60-145443, the air/fuel ratio dependent correction factor is composed of a feedback component to be derived on the basis of the oxygen concentration in the exhaust gas and a learnt component which is set at a learnt value and continuously and cyclically updated through learning process. The learnt component of the air/fuel ratio dependent correction factor includes a plurality of values set with respect to respectively corresponding engine driving range which is defined by the engine speed indicative parameter and the engine load indicative parameter. Namely, in the practical air/fuel ratio control, one of the set correction values, which corresponds to the instantaneous engine driving range, is used.
Learning process for the learnt component for updating the set data is initiated when a predetermined condition suitable for updating the learnt component data. The condition regarded suitable for learning process will be hereafter referred to as "learning condition". During learning process, the learnt component data is updated with an updating value derived on the basis of the feedback component. Basically, and as is well known, CLOSED LOOP or FEEDBACK mode air/fuel ratio control is performed at stable engine driving condition. This means that the engine driving condition indicative parameters are held unchanged or changed within relatively narrow range. Therefore, the learning process may be performed while the engine driving condition is substantially stable for allowing FEEDBACK mode air/fuel ratio control. Furthermore, in order to maintain accuracy and precision of updated value with respect to each engine driving range, it is desirable to sample the updating data when the data per se is stable. Therefore, it is usual way to perform updating operation for updating the learnt component of the air/fuel ratio dependent correction factor when the data of the engine driving range is held at one range over a predetermined number of engine cycles. The condition to be regarded suitable for updating data of the learnt component will be hereafter referred to as "updating condition".
It should be noted the word "engine driving range" used throughout the disclosure and claims means an engine driving condition as defined by the engine speed parameter and the engine load indicative parameter. For this purpose, the engine speed is divided into a plurality of engine speed ranges and the engine load is divided into a plurality of engine load ranges. One engine driving range is defined by one engine speed range and one engine load range. As set forth above, the learnt component of the air/fuel ratio dependent correction factor is set in relation to each of the engine driving ranges and is updated when the feedback component as the updating data is derived at the corresponding engine driving range.
The engine driving range is to be of such a size as to be covered with a common learnt component value. In view of precision of air/fuel ratio control, the smaller size of the engine driving range is performed since the range to be covered by a common correction value set as the learnt component. On the other hand, the smaller size engine driving range will reduce frequency of updating of the data. Namely, as will be appreciated, the engine driving condition fluctuates frequently in relative narrow range. If the engine driving range is relatively wide, the fluctuation of the engine driving condition may occur within a same engine driving range to frequently satisfy the updating condition. On the other hand, when the set size of the engine driving range is substantially small, the engine driving range may frequently vary to reduce frequency of satisfying the updating condition.